1. Field of the Invention
The present invention relates to a motor drive control apparatus and relates more particularly to a motor drive control apparatus suitable for use in a rotation polyhedral mirror drive control apparatus for an optical scanning apparatus utilized for a laser printer, a facsimile apparatus, a copying machine, a display apparatus, etc.
2. Description of the Related Art
In recent years, as environmental problems have been closed up, energy saving, ecology and recycling have come to be talked about as countermeasures for solving these problems. For this purpose, various kinds of regulations have been made at present. An optical scanning unit using such a rotation polyhedral mirror as a polygon mirror is not an exception in the discussion of such environmental problems, and various countermeasures have been made to the drive control of an optical deflector of this unit in relation to energy saving.
An optical deflector of the conventional optical scanning unit has problems of noise, a rise in temperature and the useful life of the optical deflector because a rotation polyhedral mirror is driven to rotate at a high speed. As one of countermeasures for solving these problems, there has been employed a system for stopping the rotation of a motor for driving the rotation polyhedral mirror, constituting the optical deflector, during a standby time of the optical scanning unit, not during a printing operation period thereof.
If non-contact dynamic pressure bearings are used for the bearings of a motor that drives the rotation polyhedral mirror to rotate, the bearings are brought into contact with each other when the number of revolution of the motor is decreased as at the time of stopping the rotation of the motor. Therefore, it is not so desirable to stop the rotation of the motor. Thus, there is employed a system for rotating the motor during a standby time of the optical scanning unit at a smaller number of revolution than the number of revolution at the time of the print operation. As detailed countermeasures for this, the number of the revolution of the motor is controlled to be low by dividing the frequency of an oscillation output of an oscillation circuit acting as a number of revolution command by a circuit inside the optical defector. Alternately, the motor is driven to rotate at a number of revolution which does not cause the dynamic pressure bearings to be brought into contact with each other by setting the driving voltage of the motor itself of the rotation polyhedral mirror at a lower level than the driving voltage during the printing operation, as described in the Japanese Patent Application Laid-open Publication No. 4-107520.
Further, the above-described system for stopping the rotation of the motor or for driving the motor at a low number of revolution during the standby time of the optical scanning unit has, on the other hand, a problem that the starting time of the motor becomes longer. As countermeasures for solving this problem, there has been proposed a technique for increasing the driving voltage of the motor at the motor starting time and then returning the voltage to the original or ordinary driving voltage after a lapse of a predetermined time, as described in the Japanese Patent Application Laid-open Publication No. 61-112580. There has also been proposed another technique for driving the motor at a maximum voltage of a driving power source at the time of starting the motor and then gradually lowering the voltage to a normal motor driving voltage after a pulse interval obtained by an optical synchronization detector for a deflected laser beam has become a predetermined period, as described in the Japanese Patent Application Laid-open Publication No. 61-261716.
However, when the system is employed in which the motor for driving the rotation polyhedral mirror to rotate as an optical deflector is rotated at a low driving voltage during the standby time of the optical scanning unit, a certain level of driving voltage is necessary in order to stably rotate the motor. For this purpose, the voltage needs to be set at a voltage slightly higher than the minimum driving voltage. Accordingly, although it is possible to drive the motor for driving the rotation polyhedral mirror to rotate at a lower driving voltage, this set voltage which is slightly higher than the minimum driving voltage means a waste of power.
Further, the rotation polyhedral mirrors have variations in their individual manufacturing precision. Therefore, if the driving voltage of the motor is set at a constant value, there is a possibility that some of the rotation polyhedral mirrors do not have a stable rotation.
Further, if the dynamic pressure bearings are used for the bearings of the motor as in the case of the optical deflector unit described in the Japanese Patent Application Laid-open Publication No. 4-107520, depending on the variations of the manufacturing precision of the rotation polyhedral mirrors, the bearings are always in contact with each other during the standby time so that the optical deflector may be damaged.
Further, according to the technique described in the Japanese Patent Application Laid-open Publication No. 61-261716, in the case of detecting the number of revolution of the motor by turning on a laser light source, it is at least necessary to keep the laser light source on during the detection operation of the number of revolution. This has a problem of lowering the life of the laser light source as compared with the case of carrying out the detection of the number of revolution of the motor without keeping the laser light source on.
Further, if the motor is driven to rotate at a low driving voltage which merely does not cause the bearings to be in contact with each other as shown in Japanese Patent Application Laid-open Publication No. 4-107520, there is a problem that a PLL (Phase Locked Loop) control can not be applied for the number of revolution control and thus it is difficult to detect the normal or steady rotation mode.
Furthermore, when there is a large manufacturing error in the rotation polyhedral mirrors as the optical deflectors due to the variations of their manufacturing precision, the rotation of the rotation polyhedral mirrors becomes unstable and thus the number of revolution becomes unstable. Therefore, it becomes difficult to detect the number of revolution in this case. Also, the number of revolution can not be detected if, for example, a galling is caused in the bearings for some reason, which may result in a damaging of the optical deflector.